Migration

Intermediate-distance migrant between main breeding grounds in interior (52–55°N; 58°N in sw. Alaska), and wintering areas on Atlantic and Pacific coasts between 0 and 40°N. Spring migration largely along coasts, with smaller numbers in interior (Skagen et al. 1999). Fall migration predominantly coastal, and includes 3 clear peaks on n. Atlantic coast, with breeding females preceding adult males and juveniles (Jehl 1963). Some failed or nonbreeding males join earliest flocks (Jehl 1963). Females also arrive in Florida earlier than males (Loftin 1962). Same pattern with 3 peaks of migration ob-served in Humboldt Bay, CA (Shuford et al. 1989). As Bent (1927) recognized, direct flight from vicinity of breeding grounds to n. Atlantic coast likely, as there is no time for leisurely coastal movement.

See Figure 5 .

L. g. griseus. Leaves South America from Mar to May. Large migration into Suriname without significant increase in Venezuela suggests staging along coast of Guianas, followed by nonstop flight to se. U.S. Route supported by departure bearing of 332–351° for several flocks near Suriname in late Apr and early May (Spaans 1978) and general paucity of spring records for migrants in West Indies (Raffaele et al. 1998) and Bermuda (Amos 1991).

Within U.S., migrates along Atlantic Coast, concentrating between Delaware Bay and Long Island, NY, in mid- to late May (maximum spring count 8,000 at Jamaica Bay 12 May 1939, but numbers usually peak in last week of month). International Shorebird Survey (ISS) data suggest that many arrive by direct flight from s. U.S. (B. Harrington and L. Payne pers. comm.). Numbers smaller to north (maximum spring in Massachusetts 1,400; Griscom and Snyder 1955); even scarcer in e. Canada/Maritime Provinces, indicating that flight to breeding grounds originates from New York-New Jersey area (as in hendersoni). Arrives breeding grounds in n. Quebec 18–20 May (Harris 1989).

North of New Jersey, this is the only race observed along the Atlantic Coast in spring and by far the most common one in fall (Wilds 1990). In fall, migrates along Atlantic Coast to Florida, thence south through Caribbean (Guadeloupe, Martinique, Lesser Antilles; Feldmann et al. 1999) to n. Brazil. First females depart breeding grounds in Schefferville area, Quebec (Harris 1989), and Churchill, Manitoba (JRJ), in last days of Jun and early Jul. Main southward migration of adults in Atlantic Provinces of Canada from mid-Jul to early Aug (McNeil and Burton 1973, 1977; Morrison et al. 1994). First wave usually arrives New Jersey coast around 8–10 Jul, and in Florida roughly 10 d later (Jehl 1963; cf. Loftin 1962).

Gulf of St. Lawrence is an important concentration area in fall (16,000 individuals yearly; Maisonneuve et al. 1990). Arrives Bay of Fundy in substantial numbers 30 Jun–4 Jul, with peak 30 Jul–3 Aug (largest single flock 1,100, 27 Jul 1979; total passage population estimated at 6,500–11,000; Hicklin 1987). Comparatively scarce by late Aug. Numerous in coastal Massachusetts, with several thousands regular at Monomoy and Newburyport (highest 6,000 Monomoy, 25 Jul 1958; Veit and Petersen 1993). Highest counts at Forsythe (Brigantine) National Wildlife Refuge (NWR), NJ: 11,325 in Jul 1984, 10,000 on 9 Jul 1960 (JRJ). Peak numbers smaller later in year because departure of males and juveniles from nesting areas is less synchronized.

Regular, but scarce fall migrant in Bermuda (12 Jul–3 Dec; Amos 1991). Generally arrive West Indies by Aug (Raffaele et al. 1998). Tens of thousands arrive Suriname from mid-Aug to early Oct, then move farther south (Spaans 1978). Adults arrive ne. Venezuela 5 Aug, peak 20 Sep, leave by end of Nov, return northward by Jan–Feb (McNeil 1970).

Differential migration of age and sex classes (see Jehl 1963) observed at almost all localities in fall migration, except that juveniles not detected in ne. Venezuela in fall (McNeil 1970). Migration periods of adults fairly brief (2–3 wk), whereas those of juveniles protracted. McNeil and Cadieux (1972) reported that adult Short-billed Dowitchers arrive Magdalen Is. in mid-Jul with peak 20 Jul–10 Aug, and that through the entire season males greatly outnumber females. Yet, through most of Jul, population in New Jersey is female-dominated (Jehl 1963). This suggests that postbreeding females move south faster, farther to mid-Atlantic Coast, and males make shorter first hop to coast.

L. g. hendersoni. In spring, birds from Gulf and Atlantic migrate coastally north to New Jersey before making an overland flight to breeding grounds. Low levels of occurrence in interior northeastern states in spring (e.g., New York; Levine 1998) suggests majority make a direct flight. Small numbers move northward through Mississippi Valley; largest numbers (1,500–3,500) found along Gulf Coast, scarce farther north (Skagen et al. 1999). No major spring stopover points reported in central U.S. or Canada. Peak numbers in s. Manitoba approximate only 100 (R. Koes pers. comm.). Arrive in vicinity of breeding grounds in Alberta and Saskatchewan 10–20 May (Alexander and Gratto-Trevor 1997, C. S. Houston pers. comm.); average peak number of dowitcher (spp.) migrants at Quill Lakes, Saskatchewan, was 178 (1989–1993; Alexander and Gratto-Trevor 1997).

Arrives Churchill, Manitoba, in late May and first days of Jun (Jehl and Smith 1970 and JRJ unpubl.); males average several days earlier than females. About 17–20 Jun (or about two-thirds through incubation), flocks of nonincubating birds, most females, form in evening at inland communal feeding ponds. Flock size up to about 30 in 1960s (JRJ). Other flocks forming on flats of Churchill River or Hudson Bay probably failed breeders, some immigrants from more distant locations, because too many to be accounted for solely by locals. Immigration from east evidenced by occasional appearance of adult L. g. griseus (JRJ). Flocks up to 100 at Churchill on 28 Jun 1962 (J. A. Hagar pers. comm.). Failed breeders flock and depart late Jun. This followed by postbreeding females (early Jul). By early to mid-Jul, males begin flocking inland and some present into last week of the month; Zugenruhe, other restless behavior, and arrival dates in coastal New Jersey (Jehl 1963) show they typically depart 15–20 Jul. Juveniles emigrate early to mid-Aug.

Dowitchers nesting in n. Manitoba probably move to s. James Bay (specimens Can. Mus. Nature, Ottawa) or directly (though probably not nonstop) to mid-Atlantic coast. Individuals observed departing Churchill to southeast on approximate bearing 145° (JRJ); distance to coastal New Jersey from Churchill and s. James Bay, 2,500 km and 1,360 km, respectively. The small numbers occurring in interior New York (e.g., Levine 1998), New Jersey (Walsh et al. 1999), or other eastern and northeastern states also suggest direct flight.

Migrants arrive Long Island, NY, and New Jersey 8–10 Jul, individuals stopping in coastal sites 10–14 d before moving southward (Jehl 1963). Race rare north of Long Island. In New Jersey, hendersoni makes up about 10–12% of adult migrants (Jehl 1963); increasingly common farther south and predominates south of Virginia (Wilds 1990). As many as 1,000 in flock of 10,000 at Forsythe NWR, NJ, 9 Jul 1960 (JRJ). New England records of this race concentrated in Aug and Sep (Pitelka 1950, Griscom and Snyder 1955) and mainly pertain to juveniles.

In interior of continent, slightly more common in Manitoba in fall than spring; high count of 400 on 31 Jul 1991 (R. Koes pers. comm.). No significant seasonal difference at Quill Lakes, Saskatchewan (Alexander and Gratto-Trevor 1997). Widespread but uncommon in interior states, with maximum concentration at Cheyenne Bottoms, KS (3,200; Skagen et al. 1999). In Ohio, spring flocks of 30–40, occasionally >300, along Lake Erie; in fall up to 6,400 at Winous Point (Peterjohn 1989). Maximum count in Michigan 1,300 (McPeak 1994).

In Middle America, transient and winter resident recorded mid-Jul through May in Mexico (Howell and Webb 1995), early Aug through late May in Costa Rica, with migration occurring early Aug through late Oct, and late Mar through late May (Stiles and Skutch 1989), and Aug (occasionally late Jul) through Apr in Panama (Ridgely and Gwynne 1989).

L. g. caurinus. Migrates almost exclusively along Pacific coast, where primary staging areas are localized (Paulson 1993). A few migrate through interior of w. U.S., though not specifically identified in recent study (Warnock et al. 1998). Peak numbers arrive s. and central California (San Diego–San Francisco) 10–25 Apr (Page et al. 1999). Arrive Grays Harbor, WA, mid-Apr, peaking in “tens of thousands in last week of April” (Paulson 1993: 325), then diminish within few days (Wilson 1994). Most thought to move directly from California to Alaska. Several tens of thousands use n. Gulf of Alaska coast and Prince William Sound during migration (Isleib and Kessel 1973). Peak numbers in this area Jul and Aug. Migration through Homer, AK, 27 Apr–18 May, peak 5–10 May; average number encountered/year about 2,500 (West 1996). Mixed flocks (dowitcher sp.) totaling 6,000–15,000 migrate through Yakutat Bay, se. Alaska, in spring migration (Andres and Browne 1998).

Fall migration also mainly along coast, with adults preceding smaller numbers of juveniles. Adults present in Northwest end of Jun–mid-Jul, with 12,000 at Grays Harbor, WA (10 Jul 1979, Paulson 1993). Juveniles “peak in Aug and early Sep in the low hundreds and persist in low numbers into Oct, rarely later” (Paulson 1993: 325).

Migrates in flocks by day and night during both calm and stormy weather (Brady 1994); flock size can number into hundreds or larger. Flocks of 50–60 observed at Kingston, ON, both spring and fall (Weir 1989). For historical data, see Forbush 1912; also Demography and populations: population status, below. Flocks arriving on breeding grounds average smaller; about 25 near Schefferville, Quebec (Harris 1989), about 10 at Churchill, Manitoba (J. A. Hagar pers. comm., JRJ); break up as soon as breeding habitat available (JRJ). For sex and age differences, see Timing and routes of migration, above.

Data on body mass in spring inadequate to confirm inferred flights from Middle Atlantic states to vicinity of breeding grounds (Quebec) or stopover points in s. Manitoba. Dowitchers arrive on breeding grounds in Manitoba averaging 90 (males)–105 g (females), with empty stomach and little or no subcutaneous fat. Male mass remains stable at 95–105 g into early Jul, then becomes quite variable, because some (light birds) are still attending chicks and others (heavy) are laying on premigratory fat. Female mass declines during incubation, then rises quickly by first days of Jul as females ready to leave (Fig. 2). The pattern of laying on large fat deposits before leaving breeding areas distinct from many other species of shorebirds. Weights of about 125 g probably sufficient to carry Short-billed Dowitchers (hendersoni) from Churchill area to mid-Atlantic coast without refueling. Direct (though not necessarily nonstop) flight independently indicated by correspondence of departure periods with arrivals on New Jersey coast (Jehl 1963 and JRJ). However, body mass from s. James Bay in early to mid-Jul indicates that some birds fatten there before continuing (Can. Mus. Nature, Ottawa). Extreme fat loads in griseus stopping at Gulf of St. Lawrence (Magdalen Is.) sufficient to support flight to Lesser Antilles or even ne. Venezuela, the Guianas, and n. Brazil (McNeil and Burton 1973).

Direct flight from Canadian Maritime Provinces or ne. U.S. to Florida or Caribbean suggested by similar sex ratio of dowitchers leaving Magdalen Is. and arriving in ne. Venezuela, and by sightings of migrants 50–80 miles (80–130 km) off New Jersey (Brady 1994). That shorebirds (probably including dowitchers) do make direct flight from e. Canada or ne. U.S. to e. South America is supported by radar data (Richardson 1979), by observations that fall peak numbers are reached much earlier in Suriname than ne. Venezuela (Spaans 1978), and, weakly, by band recoveries (McNeil and Burton 1973).

Earliest fall arrivals of either sex in coastal New Jersey are very lean and average 80–90 g. Female mass increases rapidly (5–6 g/d), reaching 140–150 g before they depart (see also Jehl 1963). Similar increase in males likely but not evident in Figure 2 because arrival period is protracted. McNeil and Cadieux (1972) also estimated stopover period of about 10 d. Migrants at Quill Lakes, Saskatchewan, remained 6–12 d (maximum 30); 1 juvenile gained 13.9 g in 5 d (2.8 g/d; Alexander and Gratto-Trevor 1997). Arrive Venezuela in Aug–Sep at mass of about 80 g, then increase to 90–100 g in early Nov, when apparently shift farther south (McNeil 1970: 261).

Flight capability unknown. Range estimated from energetics and fat content include: 1,900–2,000 km (Jehl 1963); average of 2,560–2,880 km (maxima 3,500–4,300) in fall from Maritime Provinces (less in spring; McNeil 1970, McNeil and Burton 1973); 2,700–3,500 km in fall from Saskatchewan (Alexander and Gratto-Trevor 1997).